The present invention relates to the installation of offshore structures; and more particularly, it relates to the installation of an offshore structure at a precise offshore location, especially in deep waters at the location of an underwater fixture.
In shallow and relatively calm waters, platform structures supporting hydrocarbon production equipment above the surface of the water are 150-200 feet in height and weigh several hundred tons. Installation of such platform structures has been generally accomplished by hoisting an assembled platform structure with a derrick barge and then lowering the structure vertically to the seabed.
When it is desired to install a fixed platform at a precise location, for example over an underwater wellhead, accurate positioning of the platform is essential. Fixed platform installation procedures, even in shallow waters, therefore, must involve steps to achieve alignment between the platform structure and the underwater fixture. Heretofore, alignment has been achieved by establishing a contacting relation between the platform and the fixture itself to locate the platform in a predetermined lateral position relative to the fixture. The method of docking an offshore structure with a submerged fixture, as disclosed by the prior art heretofore, is to position the structure above the fixture by hoisting means and then to lower the structure downwardly around the fixture with the structure being guided into a predetermined position relative to the fixture, with the necessary guidance being provided by the fixture itself. See, for example, U.S. Pat. No. 4,109,476 entitled "Docking an Offshore Structure with a Submerged Fixture."
With the production of oil and gas resources having moved into deeper waters sometimes over 1,000 feet, platform structures have correspondingly become much larger and heavier. Deep water structures in the North Sea, for example, typically weigh more than 20,000 tons. It is impossible to position such a structure above the submerged fixture with the hoisting means of a derrick barge because of the tremendous weight.
It is known in the art however that predetermined amounts of buoyancy can be built into the structure to offset all or a portion of the weight, thereby making it possible to utilize the hoisting capabilities of a floating vessel, such as a derrick barge. However, such a procedure is also attendant with difficulties when used to position a deep water structure, especially in hostile environments. The environmental forces of current and wave action can produce differential motions of the derrick barge and the suspended structure, thereby imposing severe requirements on the hoisting system. In addition, the limited capability to control lateral movement of the vertically suspended structure is a problem when positioning the structure around the template.
In order to assure that the subsea fixture is properly positioned with respect to the structure, alignment of the structure is required while lowering around the subsea fixture. The art has shown that the fixture itself can be used in a contacting means to guide the structure into the desired position, when dealing with shallower water structures. Using such a procedure with the large deep water structures presents a substantial risk of damage to the subsea fixture, especially in a hostile environment, where the structure is subjected to wind and wave action. If the subsea fixture is a template containing several drilled wells, a contacting of the template by a moving deep water structure could very well mean serious economic and environmental damage.